1. Field of the Invention
This invention relates to porous metal coated implants, such as medical prostheses in the form of hip prosthesis consisting of an acetabular cup and a femoral stem, wherein a porous metal coating is applied to the surface of a metal substrate preconfigured in the form of the medical prosthesis. The implant according to the invention is produced by diffusion bonding metal powder particles in the form of an insert or pad to the substrate below the beta-transus of the substrate by applying suitable pressure and temperature in a non-reactive atmosphere. Preferably, the substrate is a reactive metal such as commercially pure titanium or a titanium base alloy, e.g., Ti 6Al-4V ELI and the powder insert is either commercially pure titanium or it may be a titanium alloy such as Ti 6Al-4V. The particles are further preferably in the form of an insert comprising powder spheres or spheres which have been flattened by removing a portion of each sphere to provide a bonding surface of increased area for contact to the surface of the substrate. The pressure is applied, preferably, by a novel clamping mechanism which is described hereinafter.
2. Brief Description of the Prior Art
Recently, titanium and titanium alloys have experienced increased acceptance as medical implant materials, especially for medical prosthesis such as orthopedic devices in the form of knee and hip joints. Of the available grades of titanium materials, commercially pure titanium and the Ti 6Al-4V ELI alloy have become the most commonly used for implants due to the exceptional combination of properties which they possess which include easy toleration by body tissues, corrosion resistance in body fluids, high strength, low modulus of elasticity and low density.
Despite the excellent mechanical properties of titanium, orthopedic implants of titanium materials do fail, commonly because of failure brought about by undesirable stresses induced in the implant caused by loosening or separation of the implant from the bone. This type of failure is especially typical for hip prostheses because of the cyclic loading and high stresses experienced in the hip joint.
To obviate the loosening failures, improved methods of fixation of the implant to the body bone or tissues have been employed, including the application of a porous coating to the surface of the implant which permits the bone cement or ideally the bone itself to penetrate the voids in the coating to establish and maintain a strong mechanical bond with the implant. The most commonly used porous coatings are gravity sintered spherical powder, diffusion bonded metal fiber, and plasma sprayed powder coating.
Gravity sintered spherical powder coatings of commercially pure titanium or titanium based alloys, such as Ti 6Al-4V, are available and processes for their application to reactive metal substrates in medical implants are known. One such process is disclosed and claimed in U.S. Pat. No. 4,612,160 entitled "Porous Metal Coating Process and Mold Therefor" by Alfred L. Donlevy and Clifford M. Bugle, which is assigned to the assignee of the present application. That process utilizes a rigid mold wherein the metal substrate or part thereof to be coated with metal powder is disposed in the mold, the assembly is presintered in the mold, and thereafter the coated substrate portion is removed from the mold and further sintered, the sintering being carried out in a non-reactive atmosphere, such as vacuum. The sintering of these coatings involves heating the assembly to temperatures approximating 85% of the melting point such that bonding of the powder particles to each other and to the metal substrate is achieved by solid state diffusion.
Diffusion bonded metal fiber coatings have been produced from titanium wire in the form of random porous fiber metal coatings and woven porous fiber metal coatings. These are well documented in the literature by P. Ducheyne et al. in "Titanium and Titanium Alloy Prostheses with Porous Fiber Metal Coatings," from The Cementless Fixation of Hip Endoprosthesis, ed. E. Morscher, copyright 1984 by Springer-Verlag Berlin Heidelberg and in publications of Zimmer entitled "Fatigue and Porous Coated Implants," copyright 1984 and "Mechanical Testing of Porous Implant Surfaces," copyright 1985. In the Ducheyne publication, it is reported that, in the case of woven fiber coatings, by using pressure as an activator during sintering, it is possible to lower the sintering temperature significantly and that pressure sintering of fiber titanium coatings onto a Ti-6%Al-4%V substrate has been successfully done at temperatures of 800.degree. C.-925.degree. C. which are below the .alpha.+.beta. to .beta. transition temperature (975.degree. C.).
Plasma spray coatings are produced from either commercially pure titanium or titanium alloy, such as Ti 6Al-4V. In this process material of the desired composition, usually in powder form, is melted in a plasma and propelled onto the substrate to be coated. Bonding is achieved by the impact and solidification of the molten particles on the substrate. As stated in U.S. Pat. No. 4,612,160, however, the known flame spray methods, illustrated for example by Hahn U.S. Pat. No. 3,605,123, are unattractive because pore size, volume and dimensional characteristics of the coating are difficult to control.
The mechanical property requirements of the acetabular cup and the femoral stem in a hip prosthesis are substantially different. Since the acetabular cup is primarily loaded in compression, a high fatigue limit is unnecessary since tensile stresses are required to produce fatigue failure. Moreover, since the acetabular cup is supported over most of its surface, there is minimal deflection of the cup and, therefore, high ductility is not essential.
On the other hand, the femoral stem of the hip prosthesis is cyclically loaded in such a manner that substantial tensile stresses are present in portions of the stem and, therefore, high tensile strength and a high fatigue limit are required to avoid failure of the implant. In addition, high ductility is required to permit the stem to flex under load and to transfer forces to the bone. The invention is particularly applicable, therefore, to the production of a femoral stem.